This invention relates generally to medical imaging systems, and more particularly to systems and methods for polar phase encoding for magnetic resonance imaging (MRI).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field Bo), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “dipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
A variant of the well known Fourier transform (FT) imaging technique is frequently referred to as “spin-warp”. The spin-warp technique is discussed in an article entitled “Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a 2-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gx) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (Gz) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gx is incremented (ΔGx) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
In a 3-dimensional implementation of the spin-warp method phase encoding of the spin-echo signals is performed along two orthogonal axes. As described in U.S. Pat. No. 4,431,968 entitled “Method of 3-dimensional NMR Imaging Using Selective Excitation,” a thick slab of spins is excited by applying a slab-selection gradient (Gy) in the presence of a selective RF excitation pulse and then a first phase encoding gradient (Gy) along the same axis and a second phase encoding gradient (Gx) are applied before the NMR signal acquisition in the presence of a readout gradient (Gz). For each value of the Gx phase encoding gradient, the Gy phase encoding is stepped through all its values to sample a 3-dimensional region of k-space. By selectively exciting a slab, NMR signals are acquired from a controlled 3-dimensional volume.